Food products for consumption onboard commercial aircraft are located in carts and compartments within the galley area of the aircraft. These areas must be refrigerated to satisfy the public health code. Aircraft, as well as other transport vehicles, present unique challenges to cooling these food carts or other areas that need to stay refrigerated or cooled. For example, space and weight limitations need to be considered, as well as noise and heat dissipation issues.
There are many systems currently used on aircraft and other passenger transport vehicles to refrigerate food items onboard. For example, one option is the use of refrigerators, which work much like household or commercial refrigerators. Refrigerators are not used for the majority of aircraft food cooling because of the great amount of space and labor that would be required to load and unload the food from between the carts and refrigerators on each flight. Another problem with refrigerators is that heat removed from the cooling compartment of the refrigerator is expelled directly to air in the galley. This can cause excess work for the air conditioning system of the aircraft to remove this heat. Another option is the use of dry ice. However, dry ice is short-lasting and thus provides insufficient refrigeration for longer trips. Other downsides are that dry ice is a recurring cost (it has to be re-loaded before each trip or flight) that requires extra manpower/labor to load, it adds to the weight of the vehicle, and it can cause safety concerns due to its very low temperature.
Split systems (vapor cycle) with the evaporator near the food or area to be cooled, and the compressor and condenser remotely located have also been used. The evaporator is typically either a coldplate in the refrigerated space or a heat exchanger relying on a fan to drive the airflow through the heat exchanger and chilled compartment. Split systems can be unreliable, and if they leak, the leak is at a joint or seal that needs to be repaired on the aircraft itself (i.e., it is not a line replaceable unit (LRU) that can be taken off the plane and replaced, while the defective unit is repaired elsewhere). Service can be very costly because the aircraft needs to be grounded for at least about eight hours.
Although the above-described systems are potential options, the two primary methods of refrigeration currently used on board commercial aircraft are chilled liquid systems and vapor cycle refrigerating units (also referred to as air chillers). For example, some current aircraft designs achieve galley cooling by using chilled liquid systems, which chill liquid centrally on the aircraft and pump the cold liquid throughout the aircraft. The liquid used is either Galden or a propylene glycol/water mixture (50%/50%). The liquid is chilled within a vapor cycle unit and circulated via a pump and tubing through a heat exchanger in the refrigerated compartment. Air is driven through the heat exchanger and over the food using a fan. This cools the air in the compartment and causes the liquid in the system to take on heat from the air in the compartment. After passing through the compartment, the liquid mixture is warmer than when it entered, and in order to cool the liquid back down for another pass through the refrigerated compartment, it is directed to the vapor cycle unit.
Both of the fluids used in chilled liquid systems are inferior to water as a heat transfer medium due their higher viscosity and lower specific heat. Additionally, Galden is approximately $400 per gallon, which can greatly increase the cost of using such a system. Another disadvantage of chilled liquid systems is that they involve a fairly complex series of pumps and tubing. Other drawbacks of a chilled liquid system include fluid leakage, low reliability due to its complexity, and perhaps most importantly, its weight, an issue of primary concern in aircrafts. Additionally, chilled liquid systems have historically been failure prone, and since they are an interconnected system, if one component fails, it is likely that most or all of the refrigeration is lost on the aircraft. This makes maintenance more complicated because the entire system must be checked and adjusted whenever any one component is replaced. As discussed above, any maintenance that must be conducted on an aircraft system (versus on a part that can be removed, replaced, and repaired in a workshop off-site) is very costly.
One advantage of a chilled liquid system (as opposed to the air chillers discussed below) is that the rejected heat from the chilled compartments can be dissipated in spaces away from the galley. By contrast, typical air chiller designs expel heat from the chiller near (or in or above) the galleys and create excess heat in these areas, resulting in undue strain on the air conditioning system or requiring complicated ducting to remove the heat.
Turning now to air chillers (also referred to as vapor cycle refrigerating units), air chillers are a self-contained refrigeration unit that function much like a wall air conditioner. The evaporator and condenser heat exchangers, compressor, fans, valves, plumbing, and controls are all contained within one unit. The unit is placed near the chilled galley, and the chilled air is ducted in and out of the chilled compartment. This design is still used in the majority of large commercial aircraft today. It has been preferred because of the modular nature of an air chiller—if an air chiller fails, it is removed and replaced without any impact on a larger system. Furthermore, the working fluid of an air chiller is air. If a duct seal is not installed correctly, or degrades over time, the loss of a small amount of chilled air makes the system slightly less efficient, but does not render the system inoperable.
One primary drawback of air chiller units, however, is that heat is rejected near the point at which they are installed. This can limit installation design because the units must be installed near the chilled compartments, yet they dissipate heat in the same area. The space near a galley is normally filled with other equipment and is not ideal for dissipating heat. The dissipated heat puts an additional load on the ECS (Environmental Control System) equipment, which causes inefficiency. Additionally, the galleys are near passengers, and noise from the air chiller fan and compressor can cause passenger complaints. Accordingly, the airflow used for the discharge heat must often be ducted some distance away from a chiller for heat and/or noise control, which puts an additional load on the chiller fan and requires additional space in the aircraft for the ducting. The ducting used for the chilled air and the condenser air is much larger than the tubing used for chilled liquid systems, and can be as large as six to eight inches in diameter. It can thus be challenging to locate this ducting within the aircraft system.
Although potable water systems have been tangentially suggested for use as a liquid coolant or for use in conjunction with a liquid cooled condenser, those applications describe a point-of-use heat exchange system, meaning that the system is placed in the galley food cart area. This adds to the above-described space considerations and challenges.
Accordingly, there is a need for an improved galley cooling system for aircraft or other transport vehicles that can efficiently cool a food cart or other compartment or device. Beneficial features of such an improved system are that it be low in weight, cost, and complexity, and high in efficiency, reliability, and ease of repair.